This invention relates to a pressure-sensitive conductive rubber material and, more particularly, to such rubber material having an excellent maintained elasticity. The material includes an inorganic filler in a rubber matrix and exhibits a decrease in the electrical resistance thereof when pressurized from a nonpressurized state. The resistance value varies sensitively with changes in the pressurizing force.
It is known, for example in Japanese Patent Laid-open Nos. 58504/1980, 147547/1980 and 5840/1981 official gazettes, and U.S. Pat. Nos. 2,951,817 and 3,758,213, that electrically insulative rubber containing carbon black or metallic particles reacts to deformation under pressure to provide a variable electric resistance.
It is also known, for example in Japanese Patent Laid-open No. 152033/1983 official gazette, that a dispersion of conductive magnetic metallic particles in an elastic electrically insulating polymer may be molded while applying a magnetic field in a predetermined direction, before or during the crosslinking to produce a pressure-sensitive conductive rubber in which the metallic particles are arranged along the magnetic field in a predetermined direction.
When a pressure-sensitive conductive rubber sheet of this type is distorted by a pressing force, there is a substantial probability of the conductive particles contacting each other and thereby reduce the resistance value of the material.
Another form of pressure sensitive sheet material comprises a rubber sheet having a cellular structure on the surface of the sheet. A conductive material, such as metallic powder, is provided therein to improve the sensitivity of the rubber sheet. Such a sheet is disclosed, for example, in Japanese Patent Laid-open No. 20981/1983 official gazette.
Various conductive rubber sheets in which metallic fibers are provided extending in the thicknesswise direction of the sheet (e.g., Japanese Patent Laid-open No. 220307/1983 official gazette) are also known. When such a conductive rubber sheet is pressed by electrode plates at opposite sides of the sheet, the surface of the rubber sheet contacting the electrode plates gradually increases with the result that its resistance value decreases.
However, the known pressure-sensitive conductive rubber materials have a serious disadvantage in that the probability of contact between the conductive particles varies with the temperature of the matrix. Further, the conductive particles may separate due to the compressive deformation of the rubber sheet thereby making it difficult to obtain a stable resistance value.
When such known conductive rubber material is subjected to large strains repeatedly for a long period of time, the surface portion hardens from fatigue and the rubber loses creeping resistance. As a result, the electrode plates contact the conductive materials in the surface portion and the electric resistance value prior to force being applied to the plate contacting the surface is decreased so that the desired variation in the electric resistance value proportional to the pressurizing face is not attained. It is therefore impossible to maintain desirable pressure-sensitive performance over a long period of time.
In addition, even though the resistance value of the known pressure-sensitive conductive rubber decrease proportional to the increase in pressing force, when the pressuring force arrives at a predetermined value, the resistance value decreases rapidly resulting in poor pressure sensitive performance.
The relationship between the pressing force and the resistance value of known pressure-sensitive conductive rubber is not a predetermined property thereof. Since the variation of the resistance value in a given range of the pressing force is small resulting in poor pressure-sensitive performance, these known rubbers cannot be used in applications as pressure-sensitive sensors.
Further, other known conductive rubber sheets have complicated surface structures which cause metallic particles or fiber therein to sink from the surface of the rubber matrix causing recesses on the surface. In the absence of a pressing force, the rubber sheets may inhibit current flow since the metallic filler does not directly contact the electrode plates. The electrode plates will, however, contact the metallic filler due to the deformation of the rubber matrix when the rubber sheet is subject to a pressurizing force which reduces its electric resistance value. However, in the latter instance, the resistance value does not vary until the rubber matrix is deformed sufficiently so that the electrode plates are in contact with the metallic fillers. Additionally, the resistance of these known rubber sheet has a tendency to abruptly decrease even with only a small change in pressurizing force due to small foreign materials such as dust being interposed within fine recesses on the surface of the rubber matrix, thereby reducing the electric resistance value when subject to the pressurizing force, causing the sensitivity to be decreased.